1. Field of the Invention
The present invention relates to an organic electroluminescent display device having high efficiency and a long life cycle, more particularly, to an organic electroluminescent display device having a low driving voltage and a superior efficiency and life cycle characteristics by improving an existing emission layer.
2. Description of the Related Art
Recently, an organic electroluminescent display device has been of interest as a next generation display device owing to its merits of a thin shape, wide viewing angle, light weight, miniaturization, fast response speed and low power consumption as compared with a cathode-ray tube (CRT) or a liquid crystal display (LCD). Particularly, an organic electroluminescent display device has the advantage that it can be easily fabricated through a simple fabrication process as it is simply structured of an anode, an organic film layer and a cathode. The organic film layer has various layers, depending on the functions. However, the organic film layer generally has a hole injection layer, a hole transport layer, an emission layer, an electron transport layer and an electron injection layer.
FIG. 1 is a drawing schematically illustrating structure of an ordinary organic electroluminescent display device. A hole is injected from the anode that is a transparent electrode 7 so that the injected hole is transported to an emission layer 4 through a hole injection layer 6 and a hole transport layer 5. An electron is injected from the cathode 1 is transported to the emission layer 4 through an electron injection layer 2 and an electron transport layer 3. The transported electron and hole are joined with each other to emit light.
The emission layer 4 is formed in a structure in which a dopant is doped on a host to transport the electron and the hole to the dopant through the host so that the light is emitted. In case of a phosphorescence organic electroluminescent display device, a phosphorescent material comprising Iridium (Ir) or Platinum (Pt) is used as a dopant.
As a way of improving the efficiency and the life cycle of an organic electroluminescent display device, the doping and the host of the emission layer are improved. For instance, U.S. Pat. No. 6,392,250 shows mixed layer of the hole transport layer 5 and the electron transport layer 3 is used as a host for the dopant to improve the life cycle of the organic electroluminescent display device. In addition, U.S. Pat. No. 6,285,039 shows a method for using a mixed layer of the hole transport layer 5 and the electron transport layer 3 as the host and constructing the emission layer 4 of the two layers to improve luminance of an organic electroluminescent display device.
A method for adding an intersystem crossing agent to the emission layer 4 consisting of the host and the dopant to improve emission efficiency of an organic light emitting device is shown in U.S. Pat. No. 6,310,360. Specifically, a fluorescent material in which the absorption spectrum of the dopant is overlapped with the emission spectrum of the intersystem crossing agent is used as the dopant. A phosphorescent material is used as the intersystem crossing gent to improve emission efficiency of the organic light emitting device. The phosphorescent material used as the intersystem crossing agent is fac-tris(2-phenylpyridine)iridium(Ir(ppy)3), and 3.3% of external quantum efficiency is obtained by using a fluorescent dopant DCM2 as an emitting material. In the above device, energy of a singlet or a triplet transported from the host by using the phosphorescent material as the intersystem crossing agent is transported to the fluorescent dopant so that efficiency of the fluorescent dopant is improved.
However, the foregoing methods are limited as they are applied to the fluorescent material using a singlet energy state only. The foregoing methods have problems of deposition process difficulty and device stability since three types of material have to be co-deposited at the same time. Therefore, a new device structure for having high emission efficiency and stable device characteristics is required.
Furthermore, technology using the intersystem cross agent has a problem in that color purity of the emitting material is deteriorated by an emission peak of the intersystem crossing agent as the emission of the intersystem crossing agent can not be disappeared completely.